Refer to figure.
Engines can be classified according to their design characteristics. Most commonly by the:

• Number of cylinders;
• Cylinder arrangement;
• Method of cooling;
• Type of fuel and air induction system;
• Rotor drive arrangement.

Some of the most common engine arrangements are:

• "In-line engines." The cylinders are positioned in line.
• "Radial Engines". The cylinders are arranged radially around a central crankshaft
• "V" Engine. It is effectively two in-line engines set at an angle to each other and joined by a single crankshaft.
• "Horizontally opposed" engine or "flat engine" where the cylinders and pistons are arranged equally on either side of a common crankshaft. This is the most common multicylinder arrangement used in small aircraft.